VERY interesting, I've never noticed this. The ones on my 1982 6" brushed stainless Python are like the ones on your 1986, very grainy with a "sharp" top shoulder.
I'll have to compare them to my other Pythons with 3rd gen stocks. I have a 1980 and a 1978 blued I can compare with. I've really never put them together for a comparison.
Here's the 1978 - biggest picture I have here. They look "softer" more rounded and not as grainy.
Let me dig through the safe this weekend and see what I come up with.
Have fun and be safe.